Battery charges provide charge current to batteries in a wide variety of applications. Generally, in these applications, rechargeable batteries provide the energy for operating many electronic communication devices such as portable two-way radios and pagers.
One of the popular types of rechargeable batteries is a Nickle-Cadmium (Ni-Cad) battery. The Ni-Cad batteries may be charged fairly rapidly by application of large charge current to their terminals. The Ni-Cad batteries are also capable of sourcing large current to the electronic device after they are charged, while being packaged in substantially small enclosures. Conventionally, a Ni-Cad battery may be charged in one of two modes: a rapid charge mode or a trickle charge mode. In the rapid charge mode, the battery charger supplies the battery with a steady charge current for a substantially short period of time (typically 1 to 2 hrs). In the trickle charge mode, the battery charger supplies a small charge current generally to prevent the battery from discharging and losing its maximum capacity. It is, therefore, customary to rapid charge the battery to its near full capacity and then revert to the trickle charge mode for maintaining the battery capacity.
Under normal ambient temperature conditions (e.g., at room temperature), a depleted battery absorbs most of the energy provided by the charge current. Thus, at the start of the charge cycle, the voltage across battery increases rapidly as the charge current energy increases the battery capacity. However, when the battery is charged to near full capacity, the energy provided by the charge current is not as efficiently absorbed and the battery voltage starts to level off. The excessive energy not absorbed by the battery is dissipated in the form of heat which also increases the battery temperature. Therefore, as a result of excessive application of the charge current, the heat generated could adversely effect battery life and in extreme circumstances may even cause the battery to explode. In less extreme circumstances, the excessive charge current would destroy battery electrolytes, thereby shortening the battery life. Therefore, maintaining proper limits for application of the charge current becomes an important consideration during the charge cycle.
Conventional battery chargers include sensing means for terminating charge current supply upon completion of the charge cycle. The methods of sensing the end of charge cycle usually entail monitoring one of either the battery voltage or the battery temperature. In one conventional approach known as delta voltage scheme, the battery charger continuously measures the voltage across the battery terminals in order to determine the rate of change of the battery voltage. During the rapid charge mode, the battery charger compares successive rate of change determinations and upon detection of a certain rate of change threshold, the rapid charge mode is terminated.
Another known conventional approach is a delta temperature scheme where instead of monitoring the battery voltage, the battery charger monitors the rate of change in battery temperature. In this approach, a thermistor disposed within the battery package provides the battery charger with information relating to the battery temperature. The battery charger determines the rate of change of the battery temperature by successive comparison of the temperature measurements. Again, when the rate of change exceeds a threshold, the rapid charge mode is terminated. As an additional safety feature, some chargers would also prevent a battery from being charged all together, if the battery temperature is above a maximum temperature.
Although the delta voltage and delta temperature schemes described above work well under normal operating environments, at high temperature environments, such as those existing in hot deserts, the chargers may not function as adequately. This is because at high temperatures, the rate of change of the battery voltage and the battery temperature level off, making it hard to detect any rate of change. Therefore, the charger could fail to detect the threshold change required for terminating the charge current. At other instances, the ambient environment temperature may be higher than maximum temperature inhibiting the battery from being charged all together.
One prior art battery charger monitors the ambient temperature and upon detection of temperatures above a certain threshold, the rapid charge mode is disabled and the battery charger enters into the trickle charge mode. This approach, however, does not address the problem of rapid charging the battery at high temperatures. Therefore, a need exists for charging the batteries at high temperatures without causing battery damage.